This invention relates to molten metal pumps. More particularly, this invention relates to an impeller suited for use in a molten metal pump. The impeller of the present invention is particularly well suited to be used in molten aluminum and molten zinc pumps. In fact, throughout the specification, numerous references will be made to the use of the impeller in molten aluminum pumps, and certain prior art molten aluminum pumps will be discussed. However, it should be realized that the invention can be used in any pump utilized in refining or casting molten metals.
In the processing of molten metals, it is often necessary to move molten metal from one place to another. When it is desired to remove molten metal from a vessel, a so called transfer pump is used. When it is desired to circulate molten metal within a vessel, a so called circulation pump is used. When it is desired to purify molten metal disposed within a vessel, a so called gas injection pump is used. In each of these types of pumps, a rotatable impeller is disposed within a pumping chamber in a vessel containing the molten metal. Rotation of the impeller within the pumping chamber draws in molten metal and expels it in a direction governed by the design of the pumping chamber.
In each of the above referenced pumps, the pumping chamber is formed in a base member which is suspended within the molten metal by support posts or other means. The impeller is supported for rotation in the base member by means of a rotatable shaft connected to a drive motor located atop a platform which is also supported by the posts.
Molten metal pump designers are generally concerned with efficiency, effectiveness and longevity. For a given diameter impeller, efficiency is defined by the work output of the pump divided by the work input of the motor. An equally important quality of effectiveness is defined as molten metal flow per impeller revolutions per minute.
A particularly troublesome aspect of molten metal pump operation is the degradation of the impeller. Moreover, to operate in a high temperature, reactive molten metal environment, a refractory or graphite material is used from which to construct the impeller. However, these materials are also prone to degradation when exposed to particles entrained in the molten metal. More specifically, the molten metal may include pieces of the refractory lining of the molten metal furnace, undesirables from the metal feed stock and occlusions which develop via chemical reaction, all of which can cause damage to an impeller and pump housing if passed therethrough.
With regard to earlier impeller designs, U.S. Pat. No. 3,048,384, herein incorporated by reference, displays a molten metal pump with a cup-like impeller having lateral openings in the sidewall for moving molten metal. Although the impeller of this design adequately pumps molten metal, it is prone to clogging when particles are drawn into the pump. More specifically, because the inlet to the impeller makes up the entire central top surface area and extends downwardly the entire depth of the radial openings to the circular base, large particles can enter the impeller but cannot exit through the smaller radial openings. Accordingly, a risk for catastrophic failure of the pump results if a large particle is jammed against the volute or the pumping chamber. In addition, small particles can slowly clog the radial openings and degrade the performance of the impeller by reducing the volume of molten metal that can be transferred.
In U.S. Pat. No. 5,586,863, a significantly improved molten metal impeller design is provided. More specifically, an impeller comprised of a spherical base, a central hub and radially directed vanes is described. This design achieves a significant advantage by providing a smaller inlet area than outlet area, which more readily passes particles without jamming and/or clogging. However, this design is slightly disadvantaged in that molten metal flow between adjacent vanes is difficult to control.
Accordingly, an impeller having low clogging characteristics, yet also providing high effectiveness would be highly desirable in the art. The current invention achieves these objectives. Moreover, the current invention achieves a number of advantages in directional forced metal flow. For example, the impeller of the current pump is not prone to clogging as in many of the prior impellers. Accordingly, catastrophic failure is much less likely to occur and the effectiveness of operation does not degrade rapidly over time. The design also achieves high strength by increasing the percentage of the body comprised of the refractory material. Furthermore, the impeller design can be prepared with relatively simple manufacturing processes. Therefore, the cost of production is low and accommodates a wide selection of materials, such as graphite or ceramics.